I'm not a lawyer, and for answers you can count on, you should see one, NOT the slip & fall injury guys, but one experienced in firearm law.
I'd say, if the son, and the Uzi (semi auto rifle, which is an "assault weapon" ONLY IN CERTAIN STATES, in the rest of the country its just a rifle) if both are in Maryland, and LEGAL IN MARYLAND, then there is really, no issue.
So, the laws you need to check, first, are Maryland laws. I don't know Maryland law, sorry.
Again, get a lawyer, to be certain there are several possibilities and differing legal aspects, depending on what applies in your specific case.
OK, based on what you said, Dad had the Uzi, all legal and registered in NJ. Dad passes on 10years ago. Wife inherits, (all legal and registered with NJ, right???)
Wife GIFTS gun to son (after 10 years), so son does NOT legally inherit the gun in NJ. here's where it starts to get more complex.
Gifts are covered under specific language
Immediate Family members are covered under (other) specific language
In state, and out of state resident transfers are covered under different language and laws. (both state and Federal)
IF, for example, Mom & son are both NJ residents when the transfer of OWNERSHIP is made, NJ laws apply. If the son possessed it in NJ, and took it with him when he moved to Maryland, Maryland laws apply.
IF the son lived in, and owned the gun in NJ (legally) and left it there when he moved, and came back for it later, and HE took it back to Maryland, Maryland laws apply. If he was a Maryland resident when he became the owner of the gun, then Federal laws apply to the transfer, and state laws apply to possession. If the gun was SENT to him in Maryland, Fed law applies, as well as Maryland.
Yes, tis a can of worms, and possibly a law(s) was broken through ignorance. GET A LAWYER, and find out.
It is possible that since the son, and the gun are legally in Maryland now, HOW it got there might not matter. On the other hand, it might, and I cannot say. Get a lawyer in MD, and see what they say. Everything we can tell you here is just (well intentioned) guesswork.
Good Luck!